Apparatus for optical disc memory with correction pattern and master disc cutting apparatus

ABSTRACT

A record carrier having a disc-shaped substrate and a recording layer for optically writing information therein is disclosed which includes a guide groove extended in the rotational direction of the record carrier so that a plurality of revolutions of the guide groove are spaced apart from each other in radial directions of the record carrier, and a pit pattern provided between adjacent guide grooves, at positions arranged at intervals in the above rotational direction, for detecting the deviation of a light spot from the center line between adjacent guide grooves. The recording/reproducing operation for this record carrier is performed in such a manner that the first tracking error signal is detected from diffracted light from the guide groove, the second tracking error signal is detected from the pit pattern, and information is recorded in and reproduced from a land formed between adjacent guide grooves in a state that a tracking operation is performed using the first and second tracking error signals. The second tracking error signal detected from the pit pattern is not affected by the tilting of the record carrier and the movement of the light spot. When a tracking signal obtained by adding the first and second tracking error signals to each other, is used in a closed-loop tracking system, a stable tracking operation can be performed. The polarity of the pit pattern is reversed on every track, and hence a polarity pit having a depth of λ/4 (where λ indicates the wavelength of a reproducing light beam) is provided in the record carrier, to reverse the polarity of the second tracking signal on every track on the basis of a signal from the polarity pit.

This application is a divisional of application Ser. No. 875,442, filedJune 17, 1986 now U.S. Pat. No. 4,949,331.

BACKGROUND OF THE INVENTION

The present invention relates to an optical record carrier such as anoptical disc and an information recording/reproducing apparatus for theoptical record carrier, and more particularly to an optical recordcarrier suited for performing a stable tracking operation and aninformation recording/reproducing apparatus for such an optical recordcarrier. Further, the present invention relates to a master disc cuttingapparatus for forming a master disc suitable for mass producing a recordcarrier in which a guide groove, header pits, and a tracking-errordetecting pit pattern are previously formed.

An apparatus for recording information in a land existing betweenadjacent pregrooves and for reproducing the recorded information isproposed in Japanese Patent Application Unexamined Publication No.58-41446. In this apparatus, the deviation of a light spot from a trackis detected by using a 3-spot tracking servo mechanism. However, thedeviation of the light spot from a track can also be detected by achange in diffracted light pattern from the guide groove In this case,there arises a problem that an offset occurs on the basis of a change inintensity distribution of diffracted light on a photodetector which iscaused by the tilting of a record carrier, the movement of the lightspot, or others. Means for reducing such a track offset is disclosed in,for example, Japanese Patent Application Unexamined Publication No.59-19250 (corresponding to U.S. Application Ser. No. 515,520 and EPCapplication Appln. No. 83107110.5).

SUMMARY OF THE INVENTION

It is the first object of the present invention to provide a recordcarrier capable of correcting a track offset which is caused by thetilting of a record carrier (that is, an optical disc) or others wheninformation is optically recorded in and reproduced from a land existingbetween adjacent guide grooves, and an information recording/reproducingapparatus for the above record carrier.

It is the second object of the present invention to provide a masterdisc cutting apparatus capable of forming a tracking-error detecting pitpattern, a guide groove and header pits in a master disc withoutwobbling a cutting light beam.

In order to attain the first object, according to the present invention,a pit pattern for detecting the deviation of a light spot from a trackwithout being affected by the tilting of a record carrier (that is, anoptical disc), the movement of the light spot, or others, is previouslyformed in the record carrier at positions arranged at intervals in adirection in which information is recorded and reproduced, an errorless,tracking-error signal is intermittently detected from the pit pattern,another tracking error signal is obtained by using diffracted light froma guide groove which is previously formed in the record carrier, and astable, accurate tracking operation is performed in a wide frequencyrange from a high frequency to a low frequency by using the two trackingerror signals.

In a record carrier according to the present invention, a guide groovewhich serves as an optical guide and can be optically detected, ispreviously formed spirally or concentrically, to record information in aland existing between adjacent grooves, along these grooves Thus, thecenter line of a recording track is identical with the center linebetween adjacent guide grooves. Further, a pit pattern for detecting thetrue deviation of a light spot from a track without being affected bythe tilting of the record carrier, the movement of the light spot, orothers is formed in the record carrier at positions arranged atintervals along the groove, in the form of the unevenness of the surfaceof the record carrier In other words, the pit pattern has a phasestructure. Further, each of the recording tracks is divided into aplurality of sectors, each of which has a header field in which a headersignal including a sector mark, address information and synchronizinginformation is recorded, and a recording field in which data is recordedby a user. The header signal is recorded in a land existing betweenadjacent guide grooves in the form of pits which indicates theunevenness of the surface of the land In the recording field, data isrecorded in various manners in accordance with the characteristics of arecording layer of the record carrier. In order to form a pit patterncapable of detecting the true deviation of a light spot from a track,the first portion of the pit pattern is formed in the guide groove atpositions arranged at intervals so that the first portion has the samecenter line as that of the guide groove and is different in opticalcharacteristic from the guide groove. The first portions of adjacentguide grooves are provided so as not to overlap each other when viewedin a radial direction. The first portion may be one of a mirror-likesurface portion obtained by interrupting the guide groove, a phase pit(namely, a pit with a phase structure) different in width from the guidegroove, and a phase pit different in depth from the guide groove. Whenthe light spot tracing the center line between adjacent guide groovespasses through a pair of pit patterns each formed by the first portion,the light quantity reflected from the guide grooves varies as if theguide grooves were wobbled. By using the first portion, the deviation ofthe light spot from a track can be detected without being affected bythe track offset which is caused by the tilting of the record carrier orthe movement of the light spot. Further, the second portion which isdifferent in optical characteristic from the guide groove, may be formedin a land sandwiched between the first portions, along the center lineof the land. The pit pattern thus formed in a track is opposite inpolarity to the pit pattern formed in the next track, and hence a markfor indicating which of right and left first portions is firstirradiated with the light spot, is formed along the center line betweenadjacent guide grooves so that the mark is different in opticalcharacteristic from the guide groove. Various kinds of pit patternscapable of detecting the true deviation of the light spot from a trackcenter will be explained later in detail.

An optical disc apparatus according to the present invention uses arecord carrier which has a guide groove and the above-mentioned pitpatterns formed at intervals along the guide groove, and includes meansfor detecting the first tracking error signal based upon the diffractedlight from the guide groove and means for detecting the second trackingerror signal from the pit patterns which are formed at intervals alongthe guide groove, to form a tracking servo system using the first andsecond tracking error signals, thereby correcting the track offsetcaused by the tilting of the record carrier or the movement of a lightspot. Thus, the optical disc apparatus can record, reproduce and eraseinformation in and from a land sandwiched between adjacent guidegrooves, while performing a stable, accurate tracking operation.

Further, in order to attain the second object, according to the presentinvention, there is provided a master disc cutting apparatus for formingthe master disc of a record carrier according to the present invention,in which a laser beam for forming header pits and another laser beam forforming the guide groove are simultaneously incident on a focusing lens,and are modulated independently of each other. The first portion of thepit pattern for detecting the true deviation of a light spot from atrack has the same center line as that of the guide groove, and hencecan be formed only by modulating the intensity of the laser beam forforming the guide groove. Further, the second portion of the above pitpattern is disposed along the center line of a land sandwiched betweenadjacent guide grooves (that is, the center line of a track), and hencecan be formed only by modulating the intensity of the laser beam forforming header pits Accordingly, the above pit pattern can be formed ina master disc without wobbling the laser beams, at the same time as theguide groove and the header pits are formed in the master disc. Thus, arecord carrier which has a guide groove, header pits and a pit patternfor detecting the deviation of a light spot from a track, in accordancewith the present invention, can be mass produced from the above masterdisc through the well-known replication technique.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b show the structure of a record carrier, FIG. 1a is aperspective view showing a portion of a recording layer of the recordcarrier, and FIG. 1b is a plan view showing a positional relation amonga guide groove, a header pit and a data pit which are formed in therecord carrier.

FIG. 2 is a plan view showing an example of a pit pattern which ispreviously formed in a record carrier according to the presentinvention, to detect the deviation of a light spot from a track.

FIGS. 3a, 3b and 3c are plan views showing examples of a pit patternwhich is previously formed in a record carrier according to the presentinvention in a wobbling manner, to detect the deviation of a light spotfrom a track.

FIGS. 4a to 6d are plan views showing examples of a pit pattern which ispreviously formed in a record carrier according to the present inventionwithout being wobbled, to detect the deviation of a light spot from atrack, FIGS. 4a to 4d show a case where the first portion of the pitpattern is formed along the center line of the guide groove so that thefirst portions in adjacent guide grooves do not overlap each other whenviewed in a direction perpendicular to the guide grooves, FIGS. 5a to 5dshow a case where, in addition to the first portions formed along thecenter line of the guide groove, the second portion of the pit patternis formed in a region sandwiched between the first portions provided inadjacent guide grooves, along the center line of the above region, andFIGS. 6a to 6d show a case where the first portion of the pit pattern isformed in a region where the guide groove is interrupted, in such amanner that the first portion is connected with the guide groove.

FIG. 7 is a block diagram showing an embodiment of a master disc cuttingapparatus according to the present invention.

FIGS. 8a and 8b are time charts showing examples

FIGS. 9a and 9b are time charts showing other examples of the operationof the embodiment of FIG. 7.

FIG. 10 is a schematic diagram showing an embodiment of an optical discapparatus according to the present invention.

FIGS. 11 and 12 are block diagrams showing examples of a tracking-errordetcting circuit used for a record carrier according to the presentinvention.

FIG. 13(a) and 13(b) are waveform charts showing waveforms which areobtained by the circuit of FIG. 12.

FIG. 14 is a blick diagram showing a further example of a tracking-errordetecting circuit used for a record carrier according to the presentinvention.

FIG. 15 is a block diagram showing the outline of a tracking-errordetecting system used for a record carrier according to the presentinvention.

FIG. 16 is a graph showing the gain-frequency characteristic of thesystem of FIG. 15.

FIG. 17 is a block diagram showing another embodiment of an optical discapparatus according to the present invention.

FIG. 18 is a block diagram for explaining a tracking control system.

FIG. 19 is a block diagram showing still another example of atracking-error detecting circuit used for a record carrier according tothe present invention.

FIG. 20 is a diagram showing a track structure which has both the pitpattern of FIG. 6a and the pit pattern of FIG. 6b, and signals which areobtained from the track structure by the circuit of FIG. 19.

FIG. 21 is a block diagram showing the operation of the tracking-errordetecting circuit of FIG. 19 in detail by using transfer functions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will e explained below in detail, on the basis ofembodiments thereof.

FIGS. 1a and 1b show an example of a record carrier, to which thepresent invention is applied, FIG. 1a is an enlarged perspective viewshowing the recording layer of the record carrier, and FIG. 1b is a planview showing the positional relation among a guide groove, a header pitand a data pit which are formed in the record carrier. A recording layer4 shown in FIG. 1a is formed on a disc-shaped transparent substrate, andis coated with a protective film, if necessary. Light impinges upon therecording layer 4 through the transparent substrate. Now, let usconsider a magneto-optical disc, by way of example. This disc isprovided with a recording layer which has a thickness of about 1000 Åand is mainly made of a Tb-Fe alloy to form a perpendicularmagnetization film. Information is recorded in this recording layer inthe form of a combination of upward magnetization and downwardmagnetization. A guide groove 1 is previously formed in the recordinglayer 4 spirally or concentrically so that the pitch of the guide grooveis equal to, for example, 1.6 μm. The cross section of the guide groove1 has a substantially triangular form, and the optical depth of thegroove 1 is nearly equal to λ/8 (where λ indicates the wavelength of areproducing laser beam). The exposed surface of the guide groove 1 ishigher in surface noise level than a land sandwiched between adjacentgrooves, and hence information is recorded in the land in the form of amagnetic domain 3. One revolution of the guide groove 1 is divided intoa plurality of sectors, if necessary, and each sector is previouslyprovided with a sector mark for indicating the head of the sector,address information including a track number and a sector number foridentifying the sector, and synchronizing information, if necessary.Such a header signal is formed in the land sandwiched between guidegrooves in the form of phase pits (namely, pits with a phase structure)2 as shown in FIG. lb, since the land is traced with a light spot. Theoptical depth of the pits 2 is nearly equal to one-fourth of thewavelength of the reproducing laser beam. A magneto-optical disc, inwhich the header pits are previously formed in a land sandwiched betweenadjacent guide grooves, is proposed in U.S. application Ser. No.685,123.

In a case where each of tracks whose pitch is equal to 1.6 μm, is tracedwith a light spot having a diameter of about 1.8 μm, the light spot isincident upon adjacent guide grooves, and thus the interference patternbetween the diffracted light of zeroth order and the diffracted light offirst order appears on a photodetector having two sensors. When thelight spot is accurately located on the track, the interference patternis symmetrical with respect to the track. When the center of the lightspot deviates from the center line of the track, the interferencepattern becomes asymmetric, and thus the difference between the outputsof two sensors of the photodetector is not equal to zero. The abovedifference corresponds to the deviation of the light spot from thetrack. Such a tracking-error detecting method is called a push-pullmethod. In the above detecting method, the interference pattern on thephotodetector is moved by the tilting of the record carrier or themovement of the light spot. Thus, an offset is generated between theoutputs of two sensors and gives rise to an error.

The main frequency component of the tilting of the record carrier andthe movement of the light spot, each of which causes the above offset,is the rotational frequency of the record carrier. Accordingly, anerrorless, tracking-error signal for correcting the above offset has tobe detected at a frequency twice or more higher than the rotationalfrequency of the record carrier However, the detected signal is used asa control signal in a tracking operation, and a time lag is inevitablygenerated in such an operation. Accordingly, it is necessary to make thesampling frequency of the errorless, tracking-error signal five times ormore higher than the rotational frequency of the record carrier. Thatis, five or more portions each for detecting the errorless,tracking-error signal have to be provided on one track (corresponding tothe circumference of the record carrier) at regular intervals.

A header field is suited for such portions Now, explanation be made of acase where pit pattern for generating an errorless, tracking-errorsignal is formed in the header field However, the above pit pattern maybe formed at any position, provided that the advantage of the presentinvention is obtained

FIG. 2 shows an embodiment of a record carrier, in which pit patternsfor detecting an errorless, tracking-error signal are formed atintervals along a track. In more detail, FIG. 2 shows part of a headerfield of the embodiment. Referring to FIG. 2, adjacent guide grooves 1are interrupted at corresponding portions, and a pit 5 for detecting thedeviation of a light spot from a track by the heterodyne detectionmethod is formed between the portions where the guide grooves 1 areinterrupted, so that the center line of the pit 5 is placed on thecenter line between the above portions. Like the header pit 2, the pit 5has a phase structure. The edge of the pit 5 is spaced apart from theedge of the interrupted guide groove 1 so that a diffraction patternobtained when a light spot reaches the edge of the pit 5, is notaffected by the edge of the interrupted guide groove 1. Incidentally, anoptical disc (that is, the record carrier) is moved in the direction asindicated with an arrow. In other words, the light spot moves along thecenter line between adjacent guide grooves, from left to right. The samemovement as mentioned above will be made in the following embodiments.In the present embodiment, the pit 5 is formed at the end of the headerfield. However, the pit 5 may be formed at the head or in the middle ofthe header field.

FIGS. 3a to 3c show other embodiments of a record carrier according tothe present invention. In the embodiment of FIG. 3a, pits 6 and 7 havingthe same shape are formed on both sides of a center line 8 betweenadjacent guide grooves 1 in such a manner that the center line of eachof the pits 6 and 7 is spaced apart from the center line 8 by an amout Δin a radial direction, and the pits and 7 do not overlap each other whenviewed in the radial direction (that is, a direction perpendicular to adirection, in which information is recorded and reproduced). It ispreferable that the optical depth of each of the pits 6 and 7 is madeequal to that of the header pit 2 (namely, λ/4). In FIG. 3a, amirror-like surface may be arranged between the pits 6 and 7. Accordingto the present embodiment, the errorless, tracking-error signal can bedetected by the wobbling detection method using the pits and 7. In orderto prevent the above signal from being affected by the guide groove 1,the guide groove is interrupted at regular intervals, and the pits 6 and7 are formed between portions where adjacent guide grooves areinterrupted. Further, it is preferable to set the positional relationbetween the pits 6 and 7 and the edges of the interrupted guide groovesso that when a light spot reaches or leaves the edge of each of the pits6 and 7, the above signal is not affected by the guide grooves. However,in a case when only a continuous guide groove can be formed by a groovecutting apparatus, a guide groove which is not interrupted, may be used.Further, it is preferable to separate the pits 6 and 7 from each otherwhen viewed in a radial direction so that when the light spot lies on atleast a portion of one of the pits 6 and 7, the other pit is notirradiated with the light spot. Further, it is preferable from thestandpoint of signal-to-noise ratio to make the amount Δ nearly equal toone fourth of the track pitch. In the embodiment of FIG. 3b, apre-wobbled pattern is used. That is, adjacent guide grooves 1' arepre-wobbled at a position corresponding to part of the head field (forexample, a gap portion) by a little amount Δ, and thus a land sandwichedbetween adjacent guide grooves is, in effect, prewobbled. In theembodiment of FIG. 3c, adjacent guide grooves 1 are wobbled at a placecorresponding to a header field in one of upward and downward directionsand are wobbled at another place corresponding to the next header fieldin the other direction, to detect the deviation of a light spot from atrack by using signals which are obtained at the two places

In the embodiments of FIGS. 3a to 3c, a light beam has to be wobbledwhen the pits 6 and 7 or the guide 1 groove 1' is formed by the lightbeam. In the following embodiments, an errorless, tracking-errordetecting signal can be detected from a pit pattern which is notwobbled. First, the embodiment of FIG. 4a will be explained below.Referring to FIG. 4a, a portion 9 different in optical characteristicfrom the guide groove 1 is provided in the guide groove 1 at positionsarranged at regular intervals in such a manner that the center line ofthe portion 9 agrees with that of the guide groove 1 and the portions 9of adjacent guide grooves 1 do not overlap each other when viewed in aradial direction, that is, in a direction perpendicular to a directionin which information is recorded and reproduced. Thus, when the lightspot tracing the center line between adjacent guide grooves passesthrough the above portions 9, the light quantity from the track variesas if each of the guide grooves were wobbled. Further, a mark 10 forindicating which of right and left guide grooves is first irradiatedwith the light spot, is previously formed between adjacent guide groovesso as to exist in front of the above portions 9. The mark 10 may beexpressed by whether a specified pit is present or not, or may beexpressed by the length of a specified pit in a circumferentialdirection. The portion 9 different in optical characteristic from theguide groove 1 may be a mirror-like surface portion which is obtained byinterrupting the guide groove 1 (as shown in FIG. 4b). Further, as shownin FIG. 4c, the portion 9 may be a thick (or thin) portion which isobtained by increasing (or decreasing) the intensity of a light beam forforming the guide groove 1. Alternatively, the depth of the guide groovemay be made large or small at the portion 9, as shown in FIG. 4d.Incidentally, in FIGS. 4a to 4d, an optical disc (that is, a recordcarrier) is moved in a direction as indicated with an arrow.

In the embodiments of FIGS. 4a to 4d, the effective wobbling quantity ofthe guide groove is equal to half of the track pitch, and hence thedetection sensitivity for detecting the deviation of the light spot froma track is low. This problem can be solved by an embodiment shown inFIG. 5a. Referring to FIG. 5a, the guide grooves 1 is interrupted atintervals, and four pits 50 to 53 are formed in a region defined by theinterrupted portions of a pair of adjacent guide grooves in such amanner that the first pits 50 and 53 are disposed on the center lines ofthe adjacent guide grooves 1, the second pits 51 and 52 are disposed onthe center line between the adjacent guide grooves 1, the pits 50 and 51overlap each other when viewed in a radial direction, and the pits 52and 53 overlap each other when viewed in another radial direction. Then,a change in reflected light quantity caused by the pits 50 and 51becomes large and maximum when the center of a light spot is placedbetween the guide groove 1, and the center line 8 between the adjacentguide grooves. Accordingly, the detection sensitivity for detecting thedeviation of the light spot from a track is greatly improved. Further,in the embodiments of FIGS. 4c and 4d, the second pit may be formed onthe center line 8 between adjacent guide grooves so that the combinationof the portions 9 and the second pit makes the effective wobblingquantity optimum.

In an embodiment shown in FIG. 5b, the first pit 56 is formed on thecenter line of one of adjacent guide grooves and the second pit 54 isformed on the center line 8 between adjacent guide grooves so that atriangular or trapezoidal shape is defined by the pits 54 and 56 and isasymmetric with respect to the center line 8. Further, a similar shapeis defined by the first pit 55 and the second pit 57. At this time, thefirst pits 55 and 56 are located so that the pits 55 and 56 do notoverlap each other when viewed in a radial direction. Then, thedeviation of the locus of the center of a light spot on a track from thecenter line 8 can be detected by comparing a signal waveform obtained ata time the light spot passes through a pit group A (formed of the pits54 and 56) with a signal waveform obtained at a time the light spotpasses through a pit group B (formed of the pits 55 and 57). That is,the above deviation can be detected by comparing those amplitudes of twosignals obtained from the pit groups A and B which are appropriatelysampled, with each other, or by comparing the width of a pulse obtainedby slicing one of the above signals at a signal level with the width ofanother pulse obtained by slicing the other signal at the above signallevel.

An embodiment shown in FIG. 5c is based upon the same thought as in theembodiment of FIG. 5b. Referring to FIG. 5c, a triangular or trapezoidalshape is formed by a pit group C including pits 58, 59 and 60, and asimilar triangular or trapezoidal shape is formed by a pit group Dincluding pits 61, 62 and 63 but is opposite in direction to thetriangular or trapezoidal shape formed by the pit group C. Like theembodiment of FIG. 5b, a tracking error signal can be obtained bycomparing signal waveforms obtained from the pit groups C and D witheach other. Incidentally, the pits 60 and 61 may be omitted from theembodiment of FIG. 5c.

An embodiment shown in FIG. 5d has signal varying regions whichcorrespond to the reversed versions of the signal varying regionsincluded in the embodiment of FIG. 5c. In other words, a triangularregion defined by the edges of pits 64 to 69 acts as the pit group C ofFIG. 5c, and a triangular region defined by the edges of pits 67 to 72acts as the pit group D of FIG. 5c.

In the embodiments of FIGS. 5a to 5d, adjacent guide grooves are madediscontinuous in the tracking error detecting region. However, in orderto avoid a problem which arises in some tracking error detectingmethods, that is, a problem that discontinuous portions of the guidegroove operates as an external disturbance for a tracking error signal,the guide groove may be made continuous. The optical depth of a pitpattern for detecting the deviation of the light spot from a track, ismade equal to λ/4, by way of example. Then, the pit pattern can beoptically distinguished from the guide groove.

In the foregoing embodiments, explanation has been made only on a pairof adjacent guide grooves and a land formed therebetween. However, apair of adjacent lands have a guide groove in common, and different pitpatterns appear at guide grooves on both sides of a land. Accordingly,pit patterns in adjacent guide grooves and lands will be explainedbelow. Pit patterns are formed so as to be symmetrical with respect tothe center line of the guide groove. Then, the pit pattern on one ofadjacent lands will be a reversed version of the pit pattern on theother land. In order to determine the polarity of a detected trackingerror, it is necessary to know how a pit pattern is arranged on a landwhich is now traced with a light spot. For this reason, the mark 10 forindicating the arrangement of a pit pattern is formed on the center lineof a land so as to be placed in front of the pit pattern.

FIGS. 6a to 6d show further embodiments of a record carrier, in which apit pattern for detecting an errorless, tracking-error is discretelyformed. In these embodiments, a guide groove is interrupted at eachheader field so that the interrupted portion forms a mirror-like surfaceportion 75, and the portions 75 at adjacent guide grooves do not overlapeach other when viewed in a radial direction. That portion 76 of one ofadjacent guide grooves which confronts the mirror-like surface portion75 at the other guide groove, is the first phase pit having a depth ofλ/4, to enhance the signal level of a signal obtained from the pitpattern and the detection sensitivity for detecting the deviation of thelight spot from a track. Further, the second phase pit 77 which has theform of a strip and is called a center pit, may be formed in a landportion which exists between a pair of first phase pits 76. Theembodiments of FIGS. 6a and 6c do not include the center pit 77, but theembodiments of FIGS. 6b and 6d include the center pit 77, which has adepth d (where 0<d≦λ/4). In these embodiments, adjacent tracks aredifferent in the arrangement of a pit pattern from each other, and areopposite in polarity of tracking-error signal to each other. In order tocope with the above difficulties, a polarity marker 10 which has a depthof λ/4 to obtain a maximum modulation degree, is formed on every othertrack. The polarity marker 10 may be disposed in a gap field whichexists between the header field and the recording field, as shown inFIGS. 6a and 6b, or may be disposed in the pit pattern, as shown inFIGS. 6c and 6d. In both cases, the same effect is obtained. Theembodiments of FIGS. 6c and 6d are made smaller in the length of thefirst phase pit 76 than the embodiments of FIGS. 6a and 6b, to providethe polarity marker 10 in a central region between adjacent first phasepits 76. The pit patterns shown in FIGS. 5a to 5d and 6a to 6dcorrespond to the pre-wobbling detection method using a wobblingdistance Δ equal to p/2 (where p indicates a track pitch). In thepre-wobbling detection method, when the wobbling distance Δ is madeequal to p/4, the detection sensitivity for detecting the deviation ofthe light spot from a track becomes maximum. However, in order todeviate a pit from a predetermined position by a distance of p/4, it isnecessary to apply two signals having different frequencies to anacousto-optical modulator/deflector asynchronously, thereby deflecting alight beam. Accordingly, a cutting apparatus for forming such a pitpattern becomes complicated in structure. In a case where information isrecorded in a land formed between adjacent guide grooves, the centerline of a pre-wobbled pit pattern is considered to be the center line ofa track, and hence it is necessary to accurately locate the center lineof the pre-wobbled pit pattern in relation to the center line of theguide groove. The pit patterns shown in FIGS. 5a to 5d and 6a to 6d area little inferior in sensitivity for detecting the deviation of thelight spot from a track to the pre-wobbled pit pattern, but can beformed only by modulating the intensity of a laser beam by anacoustooptical modulator. Further, the center line of the guide groovecan be detected from each of these pit patterns Accordingly, even when aheader field provided on a land, a little deviates from the center lineof a track, information can be recorded on the center line of the track.

Next, explanation will be made of a master disc cutting apparatus forfabricating the master disc of a record carrier according to the presentinvention.

FIG. 7 shows an embodiment of a master disc cutting apparatus accordingto the present invention. Referring to FIG. 7, a linearly-polarizedlaser beam emitted from an argon laser 11 impinges upon a modulator 12,in which the intensity of the laser beam is modulated so as to beproportional to a radius between a laser beam receiving point on a glassdisc 23 coated with a photo-resist layer and the center axis of theglass disc 23. The laser beam from the modulator 12 is divided by a beamsplitter 13 into two parts, one of which is converted by an opticalmodulator 17 into a pulsive laser beam in accordance with a header pitforming signal, and then the plane of polarization is rotated by ahalfwave plate 27 through 90°. The linearly-polarized laser beam fromthe halfwave plate 27 has the plane of polarization parallel to thepaper of FIG. 7, and passes through a polarization beam splitter 19. Theother part from the beam splitter 13 is made intermittent by an opticalmodulator 24, to interrupt the guide groove 1 in a desired manner. Thelinearly-polarized laser beam having passed through the opticalmodulator 24 is reflected from the polarization beam splitter 19. Twolaser beams from the polarization beam splitter 19 make a small angletherebetween. These laser beams pass through a collimator lens 20, andare then focused on the photo-resist layer of the glass disc 23 by afocusing lens 22. Incidentally, reference numerals 16, 18 and 21designate reflecting mirrors. Let us consider a case where the trackpitch is made equal to 1.6 μm and the focusing lens 22 has a numericalaperture of 0.9. In order to form the header pit 2 in a substantiallycentral region of a land, it is necessary to make the angle between twolaser beams incident upon the focusing lens 22 nearly equal to 0.02°.This angle is formed by deflecting the laser beam from the opticalmodulator 24 by a prism 25. When the incident angle of the laser beamfrom the optical modulator 17 on the focusing lens 22 and themagnification of the lens 20 are known, the apex angle of the prism 25for obtaining the above angle between two laser beams can be determined.Thus, a stable optical path can be readily determined.

The optical construction of the present embodiment has been explained inthe above. Next, the electrical construction of the embodiment will beexplained. Referring again to FIG. 7, the glass disc 23 is connected toa motor 81 through a motor shaft 83, and turns on a rotating axis 80.Further, a rotary encoder 82 is mounted on the axis of the motor 81, andthe output of the rotary encoder 82 is applied to a rotation controlcircuit 93, in which the output of the rotary encoder 82 is comparedwith a rotation reference signal 92, to supply a control signal to themotor 81, thereby causing the motor 81 to synchronize with the referencesignal 92. The reference signal 92 will be explained later.

The glass disc 23 and the motor 81 are mounted on a movable base 84,which is connected to a feed motor 87 through a feed screw 86. Anotherrotary encoder 88 is mounted on the feed motor 87, and the output of therotary encoder 88 is applied to a control circuit 89, to be comparedwith a feed reference signal 90. The feed motor 87 is controlled on thebasis of the result of the above comparison. Although the feed referencesignal 90 will be explained later, the rotation reference signal 92 andthe feed reference signal 90 are set so that the pitch of guide groovesis kept constant.

A scale 85 for detecting the position of the movable base 84 is attachedto the base 84, and delivers a position signal 91 which indicates theposition of the scale 85. The position signal 91 is applied to a powercontrol circuit 95 which generates a signal for controlling the power oflaser beam necessary for a recording operation in accordance with aradius between the laser beam receiving point on the glass disc 23 andthe center axis of the disc 23. The power control circuit 95 is alsoapplied with a reference clock signal from an oscillator 94, and theoutput of the power control circuit 95 controls the modulator 12 so thatthe laser beam emitted from the laser 11 has an intensity suited for arecording operation. The position signal 91 is also applied to areference signal generating circuit 96, which generates the referencesignals 90 and 92 with the aid of the reference clock signal. Thereference signal generating circuit 96 also generates an address signal97 which includes a track number and a sector number, on the basis ofthe position signal 91 and the reference clock signal.

The address signal 97 and the reference clock signal are applied to amodulation circuit 98, which generates a signal for determining a pitpattern on the guide groove. This signal is applied through a delaycircuit 100 to a drive circuit 101, which drives the optical modulator24. Thus, a modulated, guide groove forming laser beam 104 emerges fromthe optical modulator 24.

The address signal 97 and the reference clock signal are also applied toanother modulation circuit 99, which generates a signal for determininga pit pattern on the land formed between adjacent guide grooves. Thissignal is applied through another delay circuit 102 to another drivecircuit 103, which drives the optical modulator 17. Thus, a modulated,land cutting laser beam 105 emerges from the optical modulator 17. Thedelay circuits 100 and 102 are used for matching the guide track forminglaser beam 104 to the land cutting laser beam 105. In more detail, eachof the optical modulators 17 and 24 may be a modulator utilizing theelectro-optic effect or a modulator utilizing the acousto-optic effectIn either case, a time lag is inevitable in each optical modulator, andmoreover the optical modulators 17 and 24 are different in time lag fromeach other. Thus, the delay circuits 100 and 102 are indispensable formatching the laser beams 104 and 105 to each other. Now, let us considera case where a pit pattern shown in FIG. 5c is formed in a master discand a pit pattern shown in FIG. 6b is formed in another master disc, byway of example. In order to form the pit pattern of FIG. 5c, it isnecessary for the laser beams 104 and 105 to have intensity waveformsshown in FIGS. 8a and 8b. Further, in order to form the pit pattern ofFIG. 6b, it is necessary for the laser beams 104 and 105 to haveintensity waveforms shown in FIGS. 9a and 9b. FIGS. 8a and 8b show thewaveforms before and after one revolution of the disc 23, respectively.Similarly, FIGS. 9a and 9b show waveforms before and after onerevolution of the disc 23, respectively. In FIGS. 8a, 8b, 9a and 9b,reference numerals with single prime (for example, 2', 10', 58', 59',76', 77') correspond to reference numerals with no prime in FIGS. 5c and6b (for example, 2, 10, 58, 59, 76, 77), and a reference numeral withdouble prime (that is, 1") corresponds to the reference numeral 1 inFIGS. 5c and 6b. The pulse 10' corresponding to the mark 10 appears atevery other revolution. In order to wobble a pit pattern or the guidegroove as shown in FIGS. 3a to 3c, it is necessary to provide an opticaldeflector on one of two optical paths shown in FIG. 7, or to cause theacousto-optical modulator 17 or 24 to act as an optical modulator and anoptical deflector. In this case, a deflection signal is formed from theaddress signal 97 and the reference clock signal, and drives the aboveoptical deflector through a delay circuit, to match the deflectingoperation to the intensity modulation of the laser beam. Thephoto-resist layer of the disc 23 exposed to the laser beams of thecutting apparatus of FIG. 7, is subjected to developing, to removeexposed portions, thereby forming a guide groove, header pits, and pitpatterns for detecting the deviation of the light spot from a track.Thus, a master disc is completed. The master disc is subjected to theprocessing for making a surface conductive, and then electroforming withnickel, to form a stamper. A large number of disc-shaped substrates eachhaving the above-mentioned guide groove, header pits and pit patternscan be fabricated from the stamper through replication techniques. Eachsubstrate is coated with an appropriate recording layer in accordancewith a recording method, to form a record carrier.

FIG. 10 shows an embodiment of an optical disc apparatus for performingrecording, reproducing and erasing operations for a magneto-optical disc(that is, a record carrier) according to the present invention.Referring to FIG. 10, a laser beam emitted from a semiconductor laser 31impinges upon a collimator lens 32, and thus a laser beam formed ofparallel light rays emerges from the lens 32. The laser beam from thelens 32 is deflected by a triangular prism 33 so as to have a circularcross section, passes through a beam splitter 34, and then impinges on amirror 38. The laser beam reflected from the mirror 38 is focused on therecording layer of a disc 30 by a lens 35. An electromagnetic coil 49for generating a magnetic field necessary for recording and erasingoperations is disposed so that the disc 30 is interposed between thelens 35 and the coil 49. The laser beam reflected back from the disc 30is reflected from the beam splitter 34, and then impinges on a beamsplitter 36. The laser beam reflected from the beam splitter 36 passesthrough an analyzer 37, and is then reflected from a mirror M1. Thelaser beam from the mirror M1 impinges on a photodetector 39 through alens L1, and thus magnetization information and a header signal can bedetected. While, the laser beam having passed through the beam splitter36 is led into an optical system 40 for obtaining control signals whichare necessary for automatic focusing and tracking control. For example,the laser beam from the beam splitter 36 is divided by a beam splitter41 into two parts, one of which passes through a lens 48 and thenimpinges upon a photodetector 42 having a pair of sensors for detectingthe deviation of the light spot from a track, and the other part passesthrough an automatic optical system formed of a spherical lens 44 and acylindrical lens 45, and is then reflected from a mirror 46. The laserbeam from the mirror 46 is partially interrupted by a knife edge 47, andthen impinges on a photodetector 43 for detecting the out of focus. Anout-of-focus detecting system is disclosed in U.S. Pat. No. 4,450,547.The above-mentioned optical parts make up an optical head, and the wholeor part of the optical head can move in a radial direction of the disc30. Many methods for detecting the out of focus have been proposed, andall of the methods are applicable to a disc (namely, record carrier)according to the present invention.

Now, explanation will be made on how to obtain a tracking-error signalwhich is independent of the tilting of the disc or the movement of thelight spot, from the pit patterns used in the embodiments of a recordcarrier according to the present invention.

For the embodiment of FIG. 2, the pit 5 is detected by the heterodynemethod. The detection method is described in detail in Japanese patentapplication Unexamined Publication No. 58-203636, and hence only thatpart of the method which is concerned with the present invention, willbe explained below. The photodetector 42 has four sensors, which arearranged with respect to the direction of a track, as shown in FIG. 11.The outputs of two sensors on a diagonal are applied to an adder 110 toobtain the sum of the above outputs. Similarly, the sum of the outputsof two sensors on another diagonal is obtained by an adder 111. Theoutputs of the adders 110 and 111 are applied to comparators 112 and113, respectively, to be digitized. The outputs of the comparators 112and 113 are applied to integration circuits 114 and 116. In eachintegration circuit, when an input is applied to an ST terminal, theintegrating operation is started with a predetermined time constant Whenan input is applied to an SP terminal, the above integrating operationis stopped, and an integrated value is held. Thus, the integrationcircuits 114 and 116 deliver an analog quantity proportional to adifference in starting or terminating time of output between thecomparators 112 and 113. The difference between the outputs of theintegration circuits 114 and 116 is obtained by a difference circuit117, and is used as a tracking error signal 118, which is intermittentlyobtained.

Further, the sum of the outputs of a pair of sensors symmetrical withrespect to the direction of the track is obtained by an adder 119, andthe sum of the outputs of another pair of sensors symmetrical withrespect to the direction of the track is obtained by an adder 120. Theoutputs of the adders 119 and 120 are applied to a differentialamplifier 121, the output of which is used as a tracking error signal122 based upon diffracted light.

Next, explanation will be made on a case where a tracking error signalis obtained from the prewobbled pit pattern shown in FIG. 3a or theprewobbled guide grooves shown in FIG. 3b. In this case, thephotodetector 42, as shown in FIG. 12, has a pair of sensors 131-1 and1-2, which are arranged symmetrically with respect to the direction of atrack. The sum of the outputs of the sensors 131-1 and 131-2 is obtainedby an adder 130, and is applied to a timing signal generation circuit132 and sample/hold circuits 133 and 134. In the timing signalgeneration circuit 132, a synchronizing pit in the header field isdetected, and the synchronizing signal thus obtained is used for forminga timing signal necessary for obtaining a signal from the wobbled pits 6and 7. The above timing signal is applied to the sample/hold circuits133 and 134, and the outputs of these circuits are applied to adifferential amplifier 135, the output of which is used as the trackingerror signal 118.

The above synchronizing pit may be replaced by the sector mark disclosedin Japanese patent application Unexamined Publication No. 58-169337, orthe SYNC mark disclosed in Japanese patent application UnexaminedPublication No. 58-169341. Further, an elongate hole used in a compactdisc may be substituted for the synchronizing pit.

In a case where the wobbled guide grooves of FIG. 3c is used, it isnecessary that the timing signal (namely, sampling signal) from thetiming signal generation circuit 132 is not completed in one sector butcontinues for a period corresponding to two sectors, to control one ofthe sample/hold circuits 133 and 134 when the light spot passed througheach of two sectors.

In the pit patterns shown in FIGS. 4a to 4d and 5a, the polarity ofeffective wobbling is reversed at intervals of one sector, and hence thepolarization mark 10 is provided on a land. Accordingly, when the mark10 is detected by the timing signal generation circuit 132, samplingpulses applied to the sample/hold circuits 133 and 134 are exchanged.Thus, a stable, tracking error signal is obtained from two continuoussectors.

In a case where the pit patterns shown in FIGS. 5b to 5d are used, thesample/hold circuits 133 and 134 are controlled so that signal levelscorresponding to respective center portions of the pit groups A and B orsignal levels corresponding to respective center portions of the pitgroups C and D are delivered from the circuits 133 and 134.

Further, the following waveform processing method may be used. Now, letus consider a case where a light spot moves in the direction of a trackand passes through the pit pattern shown in FIG. 5c. When the light spotdeviates upwardly or downwardly from the center line 8 between adjacentguide grooves, signal waveforms shown in FIG. 13 are obtained. That is,when the light spot moves on the center line 8, a signal waveform isobtained which is indicated by a solid line in FIG. 13. When the lightspot deviates upwardly from the center line 8, a signal waveform isobtained which is indicated by a dot-dash line in FIG. 13. When thelight spot deviates downwardly from the center line 8, a signal waveformis obtained which is indicated by a broken line in FIG. 13. In otherwords, when the light spot deviates upwardly or downwardly from thecenter line 8, a time interval necessary for the light spot to passthrough the pit group C differs from a time interval necessary for thelight spot to pass through the pit group D. The difference between thesetime intervals can be detected in such a manner that waveforms shown inFIG. 13 are formed by slicing the signal waveforms of FIG. 13 at asignal level, or leading and falling edges in the signal waveforms ofFIGS. 13 are detected by differentiating the signal waveforms. When thedifference between the above time intervals is detected on the basis ofthe leading and falling edges in one of the signal waveforms, thedifference will not be affected by an electrical offset or a change inintensity of the light spot. The leading and falling edges in a signalwaveform may be detected in such a manner that the signal waveform iscaused to pass through a delay circuit, and the difference between theinput and the output of the delay circuit is taken out.

The difference between the time intervals necessary for the light spotto pass through the pit groups C and D can be converted into thetracking error signal by using the integration circuits and differentialamplifier such as shown in FIG. 11. In this case, the ST terminal ofeach integration circuit is applied with a signal which indicates afalling edge, and the SP terminal is applied with a signal whichindicates a leading edge. The whole circuit configuration for obtainingthe tracking error signal is shown in FIG. 14. Referring to FIG. 14, thesum of the outputs of the sensors 131-1 and 131-2 is applied to a timingsignal generation circuit 132', which includes both a polarity judgingcircuit 140 for detecting the polarity identification mark 10 togenerate a polarity reversing signal, and an edge detection circuit 141for generating a signal which indicates the above-mentioned leading andfalling edges. The edge detection circuit 141 supplies leading andfalling edges corresponding to the pit group A (or C) to an integrationcircuit 142, and supplies leading and falling edges corresponding to thepit group B (or D) to an integration circuit 143. The outputs of theintegration circuits 142 and 143 are applied to a polarity reversingcircuit 144, to be polarity-inverted depending on the state of theoutput of the polarity judging circuit 140. Outputs from the polarityreversing circuit 144 are applied to a differential amplifier 135, theoutput of which is used as the tracking error signal 118.

Next, explanation will be made of a control system using the trackingerror signals 118 and 122 which are obtained in the above-mentionedmanner. The above-mentioned, tracking-error detecting systems can besummarized as shown in FIG. 15. Referring to FIG. 15, a signal from thephotodetector 42 is applied to a sample/detection circuit 180 forgenerating the errorless, tracking-error signal 118 in a samplingmanner, a timing signal generation circuit 170, and a diffracted lightdetection circuit 160 for generating the tracking error signal 122 basedupon diffracted light. The timing signal generation circuit 170 suppliescontrol signals to the timing signal generation circuits 132 and 132', asample/hold circuit 150 for sampling and holding the errorless,tracking-error signal 118, and a sample/hold circuit 151 for samplingand holding the tracking error signal 122 based upon diffracted light.As to the errorless, tracking-error signal 118, the signal 118 issampled at a time the value of signal is established, and the sampledvalue is held till the next sampling time. Thus, a signal 152 isdelivered from the circuit 150. For the pit pattern shown in FIG. 2, thevalue of the signal 118 is established at a time just after the lightspot has passed through the pit 5. For the pit patterns shown in FIGS.3a to 3c, 4a to 4d, and 5a, the value of the signal 118 is establishedat a time just after the light spot has passed through the pre-wobbledor effectively-wobbled pit groups.

For the pit patterns shown in FIGS. 5b to 5d, the value of the signal118 is established at a time just after the light spot has passedthrough the pit group B or D. As to the tracking error signal 122, thesignal 122 is sampled at a time just before the guide groove isinterrupted, and the sampled value of the signal 122 is held till theguide groove again appears. Thus, a signal 153 is delivered from thesample/hold circuit 151. In a case where the guide groove is continuous,it is desirable to hold a value of the signal 122 at a place whichexists in front of a region where the errorless, tracking error signal118 is detected.

Next, explanation will be made of a deflecting-mirror control system(that is, a system for controlling a tracking control actuator), byreference to FIG. 10.

Referring to FIG. 10, an output 153 from the sample/hold circuit 151 andan output 152 from the sample/hold circuit 150 are applied to an adder156 through phase compensation circuits 155 and 154, respectively, tocombine the outputs 152 and 153. Now, let us express transfer functionson the sample/hold circuit-151 side and the sample/hold circuit-150 sideby G₁ and G₂, respectively. The low-frequency gain of the transferfunction G₂ having a low-frequency component is set so as to be 20 to 40db higher than the gain of the transfer function G₁ for the trackingerror signal 122 based upon diffracted light, as shown in FIG. 16.Preferably, the transfer functions G₁ and G₂ are made equal to eachother at a frequency of 100 to 200 H_(z). The tracking signal thusobtained from the adder 156 is applied to a mirror drive circuit 157, todrive the deflecting mirror 38, thereby performing a tracking operation.When the transfer characteristic shown in FIG. 16 is set, the electricoffset due to the tilting of the disc, or others is corrected, and astable tracking operation can be performed since the low-frequency gainof the detection system is made high. In the apparatus of FIG. 10, thetracking operation is performed on the basis of the deflection of themirror 38. However, the tracking operation is not limited to such amethod, but may be performed on the basis of the vibration of theobjective lens 35.

Next, explanation will be made of another embodiment of arecording/reproducing apparatus (that is, an optical disc apparatus)according to the present invention, by reference to FIG. 17.

The tracking operation may be performed by a two-stage servo system suchas disclosed in Japanese patent application Unexamined Publication No.58-91536 and U.S. application Ser. No. 730,165 May 20, 1985, now U.S.Pat. No. 4,607,358 (which is a continuation of U.S. application Ser. No.443,399 filed on Nov. 22, 1982). In the two-stage servo system, thewhole of an optical head is moved by a coarse servo system, and thedeflection of a mirror or the vibration of a lens is made by a fineservo system. The embodiment of FIG. 17 uses such a two-stage servosystem. That is, a linear actuator 210 makes possible the quick accessof the whole of an optical head 200 to a predetermined track in a radialdirection, and the offset is corrected by the low-frequency component ofthe tracking signal. In FIG. 17, the same reference numerals as in FIGS.10 and 15 designate like optical or electrical parts. Explanation ofcommon parts in FIGS. 10, 15 and 17 will be omitted for the sake ofbrevity. Referring to FIG. 17, the tracking error signal 153 which isbased upon diffracted light and is delivered from the sample/holdcircuit 151, passes through the phase compensation circuit 155, and isthen applied to the drive circuit 157, to drive the mirror 38 (or thelens 35) included in the optical head 200. The errorless, tracking-errorsignal 152 passes through the phase compensation circuit 154, and isthen applied to a linear-actuator drive circuit 211, to drive the linearactuator 210. Thus, the two-stage servo system is formed. When thetransfer function G₁ of a mirror (or lens) drive system and the transferfunction G₂ of a linear actuator drive system are set as shown in FIG.16, the low-frequency gain is high, and a stable tracking operation canbe performed.

Now, the operation of the tracking control system will be explainedbelow, on the basis of the embodiment of FIG. 10. In this embodiment,the operation of the control system is expressed by the block diagram ofFIG. 18. In FIG. 18, G'₁ and G'₂ indicate the transfer functions ofelectric systems for detected tracking-error signals, G'₂ the transferfunction of an electric system for the errorless, tracking-error signalwhich has been sampled, and G₀ the transfer function of the actuator.

Accordingly, the transfer functions G₁ and G₂ in the above embodimentare given by the following equations:

    G.sub.1 =G'.sub.1 ×G.sub.0

    G.sub.2 =G'.sub.2 ×G.sub.0

When an error component (that is, a track offset) due to the tilting ofthe disc or others, the movement of the track, and the movement of thelight spot are expressed by δ, x_(t) and x_(s), respectively, we canobtain the following equation: ##EQU1## where the first term on theright-hand side indicates the closed-loop characteristic of an ordinarycontrol system, and the second term indicates a residue which is basedupon the track offset due to the tilting of the disc or others. Thesecond term can be made small by decreasing the value of G₁. However,when the value of G₁ is decreased, the tracking ability is lowered.Accordingly, it is necessary to make small the second term withoutreducing the value of G₁.

The quantity δ is based upon the tilting of the disc, the movement ofthe light spot, or others, and hence has frequency components only in afrequency range which is several times higher than the rotationalfrequency of the disc. Accordingly, it is necessary to make small thesecond term in the above frequency range. If the value of G₂ is lessthan or equal to the value of G₁ in this frequency range, it will beimpossible to make the second term sufficiently small. In other words,the transfer functin G₂ is required to have a frequency characteristicsuch as shown in FIG. 16. When the quantity ##EQU2## is expressed by aformula ##EQU3## the quantity G₃ lies in an area which is enclosed withthe gain-frequency curves of the transfer functions G₁ and G₂ in FIG.16. The upper limit of a frequency range in which the second term can bemade small, is given by a frequency at which the above gain-frequencycurves intersect with each other. This frequency is determined from thefrequency component of the above-mentioned track offset.

In order for the transfer function G₂ to have such a frequencycharacteristic as shown in FIG. 16, it is necessary for thegain-frequency curve to have a steep slope. Accordingly, it ispreferable to make the transfer function G'₂ nearly equal to thetransfer function of a secondary low-pass filter while taking thesampling characteristic into consideration. The block diagram of FIG. 18which indicates the operation of a control system, is not limited to theembodiment of FIG. 10 which includes only one actuator, but isapplicable to the embodiment of FIG. 17 which includes two actuators. Inthis case, the transfer function G₀ is made equal to 1, and each of thetransfer functions G'₁ and G'₂ includes the transfer function of acorresponding actuator.

Next, explanation will be made of an operation for detecting a trackingerror signal from pit patterns of FIGS. 6a and 6b by the detectioncircuit of FIG. 19, by reference to FIG. 20. The detection circuit shownin FIG. 19 is similar to the detection circuit of FIG. 12 but isdifferent therefrom in that the polarity marker 10 provided in the pitpattern or at a position just after the header field is detected, andthe polarity of the tracking error signal 118 is reversed by, forexample, an analog switch 136. The polarity of the tracking error signal118 is reversed depending upon the presence or absence of the polaritymarker 10 or the time duration of a signal caused by the polarity marker10. Further, a signal from the least significant bit of an addresscounter included in the timing signal generation circuit 132 may be usedin place of the signal due to the marker 10. The tracking error signal118 obtained from a pit pattern and the tracking error signal 122 basedupon the diffracted light from the guide groove pass through the phasecompensation circuits 154 and 155, respectively, and are then applied tothe adder 156, to form a tracking signal for a closed-loop servo system,thereby performing a tracking operation. FIG. 20 shows signal waveformswhich appear at various parts of the above detection circuit when atrack having the pit patterns of FIGS. 6a and 6b is traced with a lightspot. Referring to FIG. 20, a track structure defined by adjacent guidegrooves and including the above pit patterns is shown in the first row,and a signal RD SIG read out from the above track structure is shown inthe second row. Further, timing signals SAMPLE PLS1 and SAMPLE PLS2 fordetecting two signal levels of the read-out signal RD SIG whichcorrespond to the pit patterns, are shown in the third and fourth rows,respectively. These timing signals are generated by the timing signalgeneration circuit 132. When the above two levels of the read-out signalRD SIG are detected by the timing signals, a signal OFFSET1 and a signalOFFSET2 are formed as shown in the fifth and sixth rows, respectively.These signals are applied to the differential amplifier 135, to obtain adifference signal therebetween. Thus, a tracking error signal OFFSETindicating the deviation of the light spot from the track is formed asshown in the seventh row. Further, the state of the polarity marker 10is detected from the read-out signal RD SIG by a timing pulse TIMING PLSwhich is shown in the eighth row and is generated by the timing signalgeneration circuit 133, and then the polarity of the tracking errorsignal OFFSET SIG is reversed depending upon the state of the polaritymarker 10. Then, the gain for the tracking error signal OFFSET SIG isadjusted, and the signal OFFSET SIG is added to the tracking errorsignal 122 based upon the diffracted light from the guide groove. Thus,a tracking signal TR is formed as shown in the ninth row of FIG. 20. Aservo system corresponding to the above operation will be expressed bythe block diagram of FIG. 21. In FIG. 21, reference symbol X_(t)designates the movement of the track, X_(s) the movement of the lightspot, δ the track offset, K_(d) the detection sensitivity for thetracking error signal based upon the pit patterns, K_(w) the detectionsensitivity for the tracking error signal based upon diffracted lightfrom the guide groove, g₁ the transfer function of the detection systemfor the tracking error signal based upon diffracted light from the guidegroove, g₂ the transfer function of the detection system for thetracking error signal based upon the pit patterns, and G₀ the transferfunction of an actuator (that is, an actuator for a galvano-mirror, or alens actuator). The ratio of X_(t) to X_(s) is given by the followingequation: ##EQU4## where the first term on the right-hand side indicatesthe closed-loop characteristic of an ordinary control system, and thesecond term indicates a residue due to the track offset. When the servosystem has a transfer characteristic such as shown in FIG. 16, thelow-frequency gain of the transfer function g₂ is 10 to 40 db higherthan the gain of the transfer function g₁. Accordingly, the track offsetis corrected, and a stable tracking operation can be performed.

A record carrier according to the present invention can be formed ofanother recordable optical disc using a recording layer in whichreversible phase transformation can be made between the amorphous phaseand the crystalline phase.

Further, a record carrier according to the present invention can also beformed of an adding type optical disc, in which holes can be formed in arecording layer on the basis of temperature rise due to lightabsorption. In this case, a high signal-to-noise ratio is obtained, andit is possible to carry out high-density recording for the optical discand to perform a high-speed reproducing operation.

Further, the probability of detection failure at a time the errorless,tracking error is detected from a pit pattern, can be reduced byarranging a plurality of pit patterns having the same shape. It isneedless to say that any pit pattern formed of a pit group which isdistributed asymmetrically with respect to the center line betweenadjacent guide grooves can be used in a record carrier according to thepresent invention.

Further, any combination of the pit patterns shown in FIGS. 2, 3a to 3c,4a to 4d, 5a to 5d and 6a to 6d, can be used in a record carrieraccording to the present invention.

As has been explained in the foregoing, according to the presentinvention, the offset component in the tracking signal can be corrected,and moreover the low-frequency gain of a tracking-signal detectionsystem can be made high. Thus, a stable tracking operation can beperformed.

Further, when a pit pattern is formed in an optical disc in accordancewith the present invention, the noise due to the optical disc can be putto a low level. Specifically, in a case where the pit pattern is formedin a recordable optical disc, from which a reproduced signal having alow signal-to-noise ratio is read out, such as a magneto-optical disc,the signal-to-noise ratio of the reproduced signal is greatly improved.

Further, a master disc cutting apparatus according to the presentinvention can readily and stably form a guide groove, a header pit and apit pattern on the same track of a master disc. A record carrier formedfrom the master disc which has been fabricated by the above cuttingapparatus, can generate a reproduced signal which is 3 db lower in noiselevel than a reproduced signal from a conventional record carrier havinga header pit on a guide groove. Further, the reproduced signal from amagneto-optical record carrier according to the present invention isabout 1 db higher in signal level than the reproduced signal from theabove conventional record carrier.

We claim:
 1. A recording/reproducing apparatus comprising:a recordcarrier having a disc-shaped substrate, a recording layer formed on saidsubstrate, first and second regions alternately arranged along therotational direction of the record carrier, guide grooves disposed in atleast said second region and extending in the rotational direction ofsaid record carrier while being spaced apart in the radial direction ofthe record carrier, a land extending in the rotational direction of therecord carrier between adjacent guide grooves, said adjacent guidegrooves serving as an optical guide for a light spot which follows on acenter line between said adjacent guide grooves, first elementsdifferent in optical characteristic from said guide grooves and beingprovided in said first region while extending on center lines of saidguide grooves so that said first elements of adjacent guide grooves donot overlap each other when viewed in the radial direction of the recordcarrier, and said first elements of adjacent guide grooves forming acorrection pattern for correcting track offset; an optical head forirradiating said record carrier with said light spot, said optical headbeing able to move in relation to said record carrier in the radialdirection thereof; first detection means for detecting a first trackingerror signal based upon diffracted light from said guide grooves; seconddetection means for detecting second tracking error signal from saidcorrection pattern in a sampling manner; and tracking control means forcontrolling the position of said light spot on said record carrier byusing said first and second tracking error signals so that said land isfollowed along the center line between said adjacent guide grooves withsaid light spot; wherein said second detection means includes means forreversing the polarity of said second tracking error signal on everyrevolution of said record carrier, a mark for identifying saidcorrection pattern is provided together with said correction pattern onevery other revolution of said land together with said correctionpattern, and said polarity reversing means is controlled by a signalwhich is obtained from said mark for identifying said correctionpattern.
 2. A recording/reproducing apparatus according to claim 1,wherein said tracking control means includes means applied with saidfirst and second tracking error signals for outputting a tracking signalwhich is used for controlling the position of said light spot on saidrecord carrier in a radial direction of said record carrier, and lightspot moving means included in said optical head for moving the positionof said light spot on said record carrier in a radial direction of saidrecord carrier, to drive said light spot moving means by said trackingsignal.
 3. A recording/reproducing apparatus according to claim 1,wherein said tracking control means includes first moving means formoving said optical head in a radial direction of said record carrier,second moving means included in said optical head for moving theposition of said light spot on said record carrier in a radial directionof said record carrier, means applied with said second tracking errorsignal for supplying a control signal to said first moving means, andmeans applied with said first tracking error signal for supplyinganother control signal to said second moving means.
 4. Arecording/reproducing apparatus comprising:a record carrier having adisc-shaped substrate, a recording layer formed on said substrate, firstand second regions alternately arranged along the rotational directionof the record carrier, guide grooves disposed in at least said secondregion and extending in the rotational direction of said record carrierwhile being spaced apart in the radial direction of the record carrier,a land extending in the rotational direction of the record carrierbetween adjacent guide grooves, said adjacent guide grooves serving asan optical guide for a light spot which follows on a center line betweensaid adjacent guide groovers, first elements different in opticalcharacteristic from said guide grooves and being provided in said firstregion while extending on center lines of said guide grooves so thatsaid first elements of adjacent guide grooves do not overlap each otherwhen viewed in the radial direction of the record carrier, and saidfirst elements of adjacent guide grooves forming a correction patternfor correcting track offset; an optical head for irradiating said recordcarrier with said light spot, said optical head being able to move inrelation to said record carrier in the radial direction thereof; firstdetection means for detecting a first tracking error signal based upondiffracted light from said guide grooves; second detection means fordetecting second tracking error signal from said correction pattern in asampling manner; and tracking control means for controlling the positionof said light spot on said record carrier by using said first and secondtracking error signals so that said land is followed along the centerline between said adjacent guide grooves with said light spot; whereinsaid second detection means includes means for reversing the polarity ofsaid second tracking error signal on every revolution of said recordcarrier, said polarity reversing means is controlled by the leastsignificant bit of a counter for counting up a number which ispreviously recorded in said record carrier to indicate an address.
 5. Arecording/reproducing apparatus according to claim 4, wherein saidtracking control means includes means applied with said first and secondtracking error signals for outputting a tracking signal which is usedfor controlling the position of said light spot on said record carrierin a radial direction of said record carrier, and light spot movingmeans included in said optical head for moving the position of saidlight spot on said record carrier in a radial direction of said recordcarrier, to drive said light spot moving means by said tracking signal.6. A recording/reproducing apparatus according to claim 4, wherein saidtracking control means includes first moving means for moving saidoptical head in a radial direction of said record carrier, second movingmeans included in said optical head for moving the position of saidlight spot on said record carrier in a radial direction of said recordcarrier, means applied with said second tracking error signal forsupplying a control signal to said first moving means, and means appliedwith said first racking error signal for supplying another controlsignal to said second moving means.